U.S. Pat. No. 6,148,520 discloses a conventional box cutter and generally comprises a blade slide and an activator, wherein the slide has a notch and the activator has a spring plate. The activator is moved linearly along a inclined axis. The user pushes a push button on outside of the housing of the box cutter to move the activator. The push button and the activator return by the force from a spring. When the activator moves forward, the spring plate is engaged with the notch so as to move the slide along a horizontal line so that the blade protrudes from the housing. When cutting a workpiece, the force applied to the blade by the user and the reaction force from the workpiece drive the blade to an extra distance which makes the spring plate be separated from the notch. By this separation, the activator can be returned and the blade is retracted into the housing by another spring when the blade removed from the workpiece.
However, the width and the space in the housing of the U.S. Pat. No. 6,148,520 has to be increased due to the inclined movement of the activator, but this makes the blade not to be protected by the housing and the blade is not stable when cutting the workpiece. Besides, the box cutter is designed for right-handed users.
U.S. Pat. No. 7,765,701 discloses a conventional box cutter and generally comprises a blade slide with a blade connected thereto. A protrusion extends from the rear end of the slide. An activator has a hook on the rear end thereof and the hook can push the protrusion of the slide so that the slide is pushed by the activator to move along the central axis of the housing. During the movement of the slide, two click spring pieces on two sides of the slide are constantly maintained below the eave-shaped guide on the inside of the housing. When cutting a workpiece, the force applied to the blade by the user and the reaction force from the workpiece make the blade and the slide pivot or swing so that one of the click spring pieces is separated from the guide of the housing and generates a sound to acknowledge the user. The hook and the protrusion are also separated from each other. Even when the user's finger still presses on the activator, the activator is maintained at the position where the blade is extended from the housing. The blade is retracted into the housing by another spring when the blade removed from the workpiece. During the processes, the activator is always located beside the protrusion so that the slide and the blade are retracted into the housing in an inclined status. The activator is then released, a spring pulls the activator backward and the protrusion of the activator is no longer stops the protrusion of the slide so that the slide and the blade return to the correct position from the inclined status.
It is noted that disclosed box cutter has a complicated structure and the inside of the housing includes an opening cutout, a constant clearance, an eave-shaped guide and a rib formed thereon. The slide is retracted in an inclined status which makes one side of the slide is easily worn out and deformed so that the balance of the slide can be easily affected. The slide and the blade return to the correct position until the activator returns to the initial position. Nevertheless, there are too many factors to affect the retraction of the slide, including fatigue of the spring, too much spring force, the friction between the slide and the housing, the click spring piece cannot be aligned with the opening cutout, or the click spring piece cannot return to the position below the guide. The fatigue of the spring and the friction between the slide and the housing may result that the hook is located beside the protrusion all the time so that the slide cannot return properly. When the blade and the slide are not correctly returned to the correct position, the user cannot observe the situation such that when the user pushes the activator again, the slide and the blade cannot protrude from the housing. Again, the box cutter is designed for right-handed users.
The present invention intends to provide a box cutter with an automatically retractable blade and the box cutter improves the drawbacks of the conventional ones.